The humoral arm of the immune response is considered to be the major protective mechanism elicited by a variety of viral vaccines. The induction of a potent neutralizing antibody response is one of the principal goals in HIV vaccine development. However, it has been remarkably difficult to ascertain what affect neutralizing antibodies may play in the containment of HIV primary infection, the natural history of disease or vaccination. We suggest that these difficulties stem from a combination of circumstances, some biologic and some technical. To overcome the technical limitations, we have developed a novel HIV-1 entry assay for measuring neutralizing antibodies (Nab). This assay is based on a reporter cell line (JC53-BL) that is sensitive to primary HIV-1 isolates, directly analyzes HIV entry steps and accurately quantifies entry in a single cycle of infection. Moreover, the assay is relatively simple and robust. Ultimately, the commercial viability will depend on whether the results produced from this entry assay have biologic, immunologic and clinical relevance. That is - whether the inhibition of HIV-1 infection by Nab in vitro, on JC53-BL cells, mimics inhibition on relevant target cells in vivo. We are encouraged by results obtained using this assay that have show (a) the emergence of resistant HIV-1 in patients receiving fusion inhibitor (T-20) monotherapy, and (b) Nab exert sufficient virus inhibitory pressure, in vivo, to result in the replacement of neutralization sensitive virus by resistant virus. Our hypothesis is - our entry assay is commercially viable and clinically relevant because it replicates important entry mechanisms of most HIV-1 strains on natural target cells. To address this hypothesis we propose: (1) To analyze the susceptibility of the entry assay's reporter cell line to infection by diverse strains of HIV-1 and (2) To determine the sensitivity of the JC53-BL-based entry assay for detecting heterologous Nab. [unreadable] [unreadable]